Much better samples (including these with numerous improvements) can be found at https://github.com/w3c/webcodecs/tree/main/samples
The samples in this repro are no longer maintained. I'm leaving it around just as a trail of breadcrumbs since this repo was mentioned in a few presentations.
This repo contains 2 samples using WebCodecs. These were presented at the IIT WebRTC 2021 conference (presentation recording).
rapid_video_painter.html (live demo)
Demuxes a video mp4, decodes, and renders the VideoFrames to a Canvas ASAP.
simple_video_player.html (live demo)
Demxues both audio and video mp4s, decodes them, and renders both with a/v synchrnoization using Canvas and AudioWorklet.
NOTE: The simple video player requires cross origin isolation to use SharedArrayBuffer. In addition to the live demo link above, you may use node server.js to play this sample locally.